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Methodology
Every number on this site comes from the standard amortization formula, computed to the cent, with rounding rules documented below. Nothing is estimated, interpolated, or fetched from a third party — the math runs in your browser.
The payment formula
For principal P, monthly rate r (annual rate ÷ 12 ÷ 100), and
n monthly payments:
payment = P · r·(1+r)ⁿ / ((1+r)ⁿ − 1)
When the rate is exactly 0%, the payment is simply P / n.
The schedule
Each month: interest = balance × r, then
principal = payment − interest (+ any extra), then the balance is reduced.
Total interest is the sum of every month's interest charge.
Rounding rules
- All currency arithmetic is performed in integer cents.
- Each month's interest is rounded half-up to the cent before being applied.
- The final payment absorbs any residual so the schedule sums exactly — principal column totals the loan amount to the cent.
Biweekly model
Half the monthly payment every two weeks (26 half-payments/year), with interest accrued
per two-week period at annual ÷ 26. This models payments posting on receipt;
servicers that batch half-payments monthly will show smaller savings.
Verification
The engine is exercised by an automated test suite against hand-computed fixtures and cross-checked against published outputs of major reference calculators. Where references disagree at the cent level (they do — rounding conventions differ), our documented rules above take precedence and the divergence is at most a few cents on the final payment.
What we deliberately exclude
Property taxes, insurance, PMI, HOA fees and closing costs are costs of owning, not costs of borrowing. Mixing them into interest figures obscures the loan itself, so our core tools isolate principal and interest. ARMs (adjustable rates) are not modeled; all calculations assume a fixed rate.
About the author
Built and maintained by Murugan Vellaichamy, an independent developer of free calculation tools. Questions and corrections: contactus@totalinterestcalc.com.